1. Field of the Invention
The present invention is in the field of treating a panel of a color cathode ray tube to prepare the panel for phosphor deposition and concerns itself essentially with minimizing the effects of reflections from interfaces in the panel which set up standing waves and create abnormalities in the uniformity of the color phosphor patterns which are deposited.
2. Description of the Prior Art
In the usual process for manufacture of a color cathode ray tube, lithographic processes are employed using photoresist compositions which are light sensitive in that they polymerize in the presence of actinic radiation. In the past, photoresist solutions consisting essentially of polyvinyl alchohol (PVA) and ammonium dichromate (ADC) have been used by all cathode ray tube manufacturers as the photoresist coating. This coating was applied to the inside face of a cathode ray tube and dried by means of infrared radiation. A shadow mask was inserted and the dry photoresist layer was exposed to actinic light so as to simulate electron beams from the electron gun. The exposure to actinic light produces a cross-linking reaction between the PVA and ADC so as to increase the molecular weight of the photoresist in the exposed area. In the unexposed area, the resist remains water soluble. By washing the panel with water, the unexposed area is selectively removed. This leaves "hard" PVA stripes where the phosphor is to be deposited eventually. A suspension of carbon or graphite particles is then deposited on the inner surface, followed by drying. Reversal or removal of the "hard" PVA stripes is accomplished by using a solution of hydrogen peroxide, an alkali metal periodate or other oxidizing agent, followed by water washing.
One of the primary problems associated with the aforementioned prior art is that the PVA-ADC photoresist responds linearly to light energy. In U.S. Pat. Nos. 3,917,794 and 4,191,571, which are incorporated herein by reference, there are described systems employing photoresist materials having reciprocity failure in the manufacture of cathode ray tubes. The reciprocity failure, of course, is in the inability of the light sensitive material to compensate for unusually long or short exposures, i.e. the response becomes non-linear. In U.S. Pat. No. 3,917,794, there is disclosed a photoresist material composed of a high molecular weight compound and a cross-linking agent together with a binding promoter. In U.S. Pat. No. 4,191,571, the photoresist composition contains a water-soluble aromatic azide compound and a photo-cross-linkable water soluble polymer.
Processes described in the afore-mentioned patents are useful to achieve "print-down", which is the production of a hardened response in the photoresist layer which is smaller than the aperture of the mask through which the energizing light passes. They do not provide a process which will function on a fine pitch line tube which exhibits a standing wave pattern due to internal reflections.
In the case of a fine pitch (short Q space) cathode ray tube, light refraction and reflection patterns are formed when the panel is exposed to actinic radiation. These patterns result at the shadow mask slit edge, the air-photoresist interface, and the glass-air interface. The reflections and refractions cause additive and subtractive light patterns to be presented to the photoresist coating. Due to the linear response of the PVA-ADC photoresist of the prior art, the stripe width was reduced where the effect was subtractive and increased where it was additive. This phenomenon resulted in a series of vertical bars on the finished cathode ray tube of varying brightness instead of the desired uniform brightness.
The phenomena which makes the photoresist of the present invention effective for reducing or eliminating the effects of the reflected and/or refracted light is its intensity cutoff. This cuttoff results because the photoresist is cross-linked by a free radical process.
The following situations can occur when light strikes a free radical synthesizer of the stilbene type, such as 4, 4'-diazidostilbene-2, 2'-sodium disulfonate:
1. The stilbene molecule can absorb a photon and form a free radical which in turn can react with the water soluble polymer resulting in polymerization.
2. The stilbene molecule can absorb a photon and by forming resonance structures, dissipate the energy and show no chemical change.
3. The free radical formed in (1) above can react with oxygen, hydrogen, water, or the like to consume the free radical and form a non-reactive compound of low molecular weight.
4. In the case of a low energy photon, the stilbene molecule may not absorb the energy and no reaction will occur.
The side reactions set forth in paragraphs 2, 3 and 4 are what give the resist of the present invention its desirable properties.
It is clear that insufficient drying of the photoresist results in an increase of the terminating side reaction set forth in paragraph 3 above. If the panel temperature changes during exposure, the permeation of air and hydrogen through the photoresist coating will change, i.e. there will be increased permeation with increased temperature. This results in a change in the probability of a terminating side reaction such as reaction 3. The side reactions which result in producing the desired effect of the present invention are those stated in paragraphs 2 and 4.
Another disadvantage of the prior art is in the area of the reversal solution used to dissolve the polymerized film. The use of peroxides, hypochlorides, dichromates, or periodate solutions as oxidizing agents requires the use of hot solutions, usually about 60.degree. C., and employing excessive times, on the order of 2 minutes. This makes processing a problem with respect to machine schedules and productivity. Also, the concentrations of the oxidizing agents used make them quite expensive. In the case of the peroxide and hypochloride oxidizing agents, containment is a major concern because if the vapors get into the air system of the screen room, adhesion problems will result on the subsequent phosphor applications and the photoprinting process.